Hitherto, polyester films typically such as polyethylene terephthalate films or polyethylene naphthalate films have been extensively used in various applications because of excellent properties such as mechanical strength, dimensional stability, flatness, heat resistance, chemical resistance and optical properties as well as excellent cost performance.
As one of the applications using the polyester films, there may be mentioned a release film for adhesives, a protective film for an adhesive layer of a polarizing plate or a release film used in a process for production of a ceramic capacitor. For example, in the case where the polyester film is used for adhesive products, an adhesive is dissolved or dispersed in various solvents, and the resulting solution or dispersion is applied onto a release film and then heated to remove the solvent therefrom and subject the adhesive to crosslinking reaction, thereby forming an adhesive layer on the release film. Thereafter, the release film is peeled off to allow the adhesive layer to exhibit its adhesion performance.
When using the polyester film in these applications, it is required that the polyester film has releasing property against an adhesive, etc. To this end, many methods for providing a release layer constituted of a silicone composition such as typically polydimethylsiloxane on the polyester film have been proposed (for example, Patent Literature 1). However, in these methods, an organic solvent is frequently used as a solvent for a coating agent that is used for forming the release layer, so that there tends to occur such a problem that a large-size facility is required for treating the organic solvent upon coating or drying. In addition, the methods also have such a problem that working environments as well as safety against explosion or fire accident owing to the organic solvent must be taken into consideration. On the other hand, solvent-free methods using no organic solvent have also been proposed (Patent Literature 2). In the solvent-free methods, there tends to occur such a problem that the viscosity of a coating solution is increased, so that it may be difficult to obtain a uniform coating film.
From these viewpoints, coating methods using an aqueous silicone emulsion have also been proposed (Patent Literature 3). However, in this case, there tends to occur such a problem that a stable uniform coating film is hardly formed from the emulsion owing to poor stability of the emulsion, and the resulting film is deteriorated in solvent resistance. When used in adhesive products, a coating layer is brought into contact with a solvent, and it is therefore important that the coating layer has good solvent resistance. For this reason, it has been demanded to provide a release film including a coating layer that has excellent solvent resistance.